A breaker marked 10 kAIC can interrupt up to 10,000 amperes of fault current under its specified voltage and test conditions; that marking is not its normal load-current rating. For a fully rated installation, the breaker’s interrupting rating must equal or exceed the available fault current at its line terminals. If the available fault current is 20.8 kA, a standalone 10 kAIC breaker is insufficient, while a 22 kAIC breaker clears the numerical comparison only after the voltage, fault-current assumptions, and equipment ratings are verified.
AIC Rating Comparison at 20.8 kA Available Fault Current
The table compares example interrupting-rating levels against an assumed 20.8 kA RMS symmetrical available fault current. These are rating levels found in manufacturer literature—not universal options for every breaker, panel, or voltage. Schneider Electric documents 10 and 22 kAIC QO variants, while its molded-case breaker literature includes additional rating levels.
| Breaker interrupting rating | Equivalent current | Standalone comparison with 20.8 kA available | Conditions before selection |
|---|---|---|---|
| 10 kAIC | 10,000 A | Insufficient | Cannot be used standalone at this fault-current level |
| 22 kAIC | 22,000 A | Numerically sufficient | Confirm the final fault-current maximum remains at or below 22 kA |
| 25 kAIC | 25,000 A | Numerically sufficient | Confirm this rating is available for the required breaker and voltage |
| 35 kAIC | 35,000 A | Numerically sufficient | Confirm the exact device rating and assembly suitability |
| 65 kAIC | 65,000 A | Numerically sufficient | Confirm the exact device rating and assembly suitability |
| 100 kAIC | 100,000 A | Numerically sufficient | Confirm the exact device rating and assembly suitability |
Every “numerically sufficient” entry assumes the stated interrupting rating applies at the actual circuit voltage and that the breaker is suitable for the equipment. A higher breaker rating does not automatically increase the panelboard or other equipment’s short-circuit current rating.
What an AIC Rating Means
AIC commonly means ampere interrupting capacity. Manufacturer labels and technical documents also use “interrupting rating” or “AIR.” The rating describes the maximum short-circuit current the protective device can safely interrupt under specified test conditions. Low-voltage molded-case circuit breaker interrupting ratings are generally expressed in RMS symmetrical amperes.
Three quantities must remain separate:
| Quantity | Meaning | Role in the decision |
|---|---|---|
| Breaker ampere rating | The breaker’s rated current, such as 20 A or 200 A | Part of load and overcurrent-protection selection |
| Breaker interrupting rating | The fault current the breaker is rated to interrupt at a specified voltage | Compared with available fault current at the breaker |
| Equipment SCCR | The short-circuit current rating of equipment or an assembly | Checked separately against available fault current and applicable protection conditions |
Interrupting rating and equipment SCCR address different parts of the installation. The breaker must interrupt the fault, while the equipment and circuit components must have appropriate short-circuit protection. Passing one check does not automatically pass the other.
Available Fault Current Is a Site-Specific Value
Available fault current is the prospective short-circuit current at a particular location in the electrical system. Its value depends on the source and circuit impedances, including utility supply characteristics, transformer impedance, conductor impedance, and contributions from motors or other sources.
A service calculation cannot simply be copied to every downstream panel. Conductor impedance changes the fault current along the circuit, and additional sources can contribute current. Calculate or document the value at the location being evaluated rather than treating a typical transformer value as a local result.
Voltage and System Conditions Matter
An interrupting rating is not a voltage-independent number. Read the manufacturer’s rating for the exact breaker catalog number at the actual system voltage; do not carry a rating from one voltage column into another. AC and DC applications also require their respective ratings.
Voltage markings can impose additional system restrictions. For example, Schneider Electric states that its 120/240 Vac QO/QOB breakers cannot be applied where line-to-ground voltage exceeds 120 Vac, whereas specified straight-rated 240 Vac QO-H/QOB-H breakers can accommodate line-to-ground voltage up to 240 Vac. A sufficient kAIC number does not override that restriction.
The fault-current study must also evaluate applicable fault types. A three-phase bolted-fault calculation is useful, but it does not establish that every phase-to-ground or single-pole interruption condition is covered. Eaton specifically identifies the need to check applicable three-phase and phase-to-ground duties.
NEC 110.9 and 110.10: Separate Checks
NEC 110.9 — Interrupting Rating
NEC 110.9 addresses equipment intended to interrupt current at fault levels. The interrupting rating at the nominal circuit voltage must be sufficient for the available fault current at the equipment’s line terminals. For a fully rated system, each overcurrent protective device meets that requirement through its individual interrupting rating.
This is why a standalone 10 kAIC breaker is not acceptable where the applicable available fault current is 20.8 kA. Reducing the normal connected load does not establish a lower available fault current: the fault-current calculation concerns the electrical source and circuit, not merely the operating load.
NEC 110.10 — Circuit and Equipment Protection
NEC 110.10 addresses the relationship among protective devices, circuit impedance, equipment SCCR, and other circuit characteristics. The installation must permit protective devices to clear faults without extensive damage to the electrical equipment. Eaton distinguishes this component-protection requirement from the interrupting-rating requirement in 110.9.
Consequently, replacing a low-AIC breaker with a higher-AIC breaker is not a complete compliance assessment. The panelboard, disconnects, controllers, and other applicable equipment still need appropriate short-circuit ratings and protection.
Use the Locally Adopted Edition
As of October 11, 2026, NFPA identifies the 2026 NEC as its current edition. That does not mean every jurisdiction enforces it; NFPA’s enforcement information shows that multiple editions remain in use across the United States. Confirm the adopted edition, local amendments, and requirements of the authority having jurisdiction (AHJ).
The NEC explanations here are paraphrases, not substitutes for the adopted code text or manufacturer instructions.
Hand Calculation: Approximately 20.8 kA at Transformer Secondary Terminals
This hypothetical example demonstrates a simplified three-phase transformer-terminal calculation. It is not utility data, a typical national value, or an approved equipment selection.
| Input or assumption | Example value | Meaning |
|---|---|---|
| Transformer rating | 750 kVA | Assumed three-phase nameplate rating |
| Secondary voltage | 480 V line-to-line | Voltage used in the three-phase formula |
| Transformer impedance | 4.34% | Assumed nameplate value, not a universal default |
| Primary source | Infinite-source approximation | Upstream source impedance is neglected |
| Fault location | Transformer secondary terminals | No secondary feeder impedance is included |
| Fault type | Three-phase bolted fault | RMS symmetrical calculation |
| Other source contributions | Excluded | No motor or generator contribution is included |
| Voltage and impedance adjustments | Excluded from the initial calculation | Evaluated separately below |
Eaton’s published method calculates transformer full-load current, then multiplies it by (100/%Z) to obtain the simplified secondary short-circuit current. The underlying technical reference is dated 2005; it supports the calculation method, not the claim that its historical code references are the current adopted requirements.
Calculation Steps
1. Calculate transformer full-load current.
For a three-phase transformer:
[
I_{\text{FL}}=
\frac{S_{\text{kVA}}\times 1{,}000}
{\sqrt{3}\times V_{\text{LL}}}
]
Substituting the assumed inputs:
[
I_{\text{FL}}=
\frac{750\times 1{,}000}
{\sqrt{3}\times 480}
\approx 902.1\text{ A}
]
2. Convert percent impedance to per-unit impedance.
[
Z_{\text{pu}}=\frac{4.34}{100}=0.0434
]
3. Calculate the simplified three-phase fault current.
[
I_{\text{SC}}=
\frac{I_{\text{FL}}}{Z_{\text{pu}}}
\frac{902.1}{0.0434}
\approx 20{,}786\text{ A}
]
[
I_{\text{SC}}\approx 20.8\text{ kA RMS symmetrical}
]
4. Compare the result with ratings at the applicable voltage.
A 10 kAIC breaker fails the standalone numerical check. A 22 kAIC breaker exceeds this initial result, but that comparison is not final until the study’s maximum conditions and the equipment requirements are established.
Keep percent impedance and per-unit impedance distinct. Dividing by 4.34 instead of 0.0434 produces a result 100 times too low.
Sensitivity: A Lower Impedance Can Exceed 22 kA
Fault current increases as transformer impedance decreases. As a separate hypothetical sensitivity case, assume the applicable minimum impedance is 10% below the initial 4.34% value:
[
%Z_{\text{minimum}}=4.34\times 0.90=3.906%
]
[
I_{\text{SC}}=
\frac{902.1}{0.03906}
\approx 23{,}095\text{ A}
23.1\text{ kA}
]
Under that assumption, 22 kAIC is no longer sufficient for standalone application. The 10% reduction is an explicitly assumed sensitivity input here—not a tolerance to apply automatically to every transformer. Obtain the applicable impedance tolerance from the transformer manufacturer or utility. Eaton’s calculation reference explains why impedance tolerance, voltage variation, and motor contribution can change the result.
The infinite-source approximation neglects upstream impedance, but it does not compensate for omitted motors, other sources, or unexamined fault types. Likewise, a transformer-terminal result is not a downstream-panel result unless the intervening circuit has been evaluated.
Use the site’s Fault Current Calculator alongside the hand calculation, checking its supported inputs, fault location, and assumptions before comparing results. A calculator result is not NEC approval, AHJ acceptance, or a replacement for a complete short-circuit study.
When Available Fault Current Exceeds the Breaker Rating
The corrective approach depends on whether the shortfall concerns the breaker, the equipment assembly, or both.
| Approach | Application | Essential verification | Limitation |
|---|---|---|---|
| Fully rated equipment | Each protective device has sufficient individual interrupting rating | Device ratings at actual voltage; equipment SCCR and protection | Higher-AIC breakers alone do not resolve inadequate assembly ratings |
| Tested, listed series-rated combination | A specified upstream device protects a specified downstream breaker | Exact devices, equipment listing, combination rating, markings, and applicable restrictions | Cannot be inferred from the upstream breaker’s AIC |
| Engineered evaluation of an existing installation | Existing equipment may qualify for an engineering-based series application under applicable code provisions | Qualified licensed professional engineer’s analysis, documentation, and required markings | Not a field assumption or generic substitution |
| Engineered system changes | Changes to source or circuit characteristics may alter fault current | Recalculated duties and protection throughout the affected system | Requires design review rather than a simple breaker-rating comparison |
Eaton describes both fully rated systems and series-rated applications, including engineering provisions for existing installations. Its series-rating reference is historical technical guidance; verify the requirements in the locally adopted NEC and current equipment documentation before applying any option.
Series Ratings Require an Exact Combination
A series rating belongs to a documented combination of devices and equipment—not to an arbitrary pair of breakers. A 65 kAIC main breaker does not automatically make every downstream 10 kAIC breaker suitable for 65 kA available fault current. The manufacturer’s documentation must identify the permitted combination and its conditions.
The evaluation also includes motor-contribution restrictions, required markings, and replacement-device identification. Series ratings do not establish selective coordination, and the need for upstream operation can affect continuity of service. Those consequences matter particularly where only the faulted circuit must be disconnected.
Common pitfall: Upstream current limitation is not permission to “subtract” current from a downstream breaker’s required rating. Use the documented series-rated combination or an applicable engineered evaluation; do not treat a high-AIC main breaker or a let-through curve alone as approval.
AIC Selection Checklist
Use this checklist to Verify the documentation before purchasing or approving equipment:
- Identify the actual system voltage, grounding configuration, and applicable fault types.
- Obtain utility source information and transformer data for the installation.
- Establish available fault current at the equipment’s actual location.
- Include applicable voltage variation, impedance tolerance, and source contributions.
- Check the exact breaker’s interrupting rating at the operating voltage.
- Check equipment SCCR and applicable short-circuit protection conditions separately.
- If using a series rating, confirm the exact permitted devices, assembly, markings, and restrictions.
- Evaluate selective-coordination requirements separately from interrupting capability.
- Confirm the locally adopted NEC edition and AHJ requirements.
- Reassess the study after relevant source, transformer, feeder, or equipment changes.
These checks connect the fault-current calculation to the manufacturer’s application conditions and equipment-protection requirements. They do not call for opening energized equipment; have qualified personnel obtain inaccessible nameplate or equipment information using appropriate safe-work procedures.
Official Resources
- NFPA 70: National Electrical Code — official code information and current-edition identification.
- NFPA NEC Enforcement Maps — adoption information; confirm project-specific requirements locally.
- Eaton: Short-Circuit Current Ratings — interrupting ratings, equipment SCCR, and NEC 110.9/110.10 context.
- Eaton: Short Circuit Current Calculations — transformer and point-to-point calculation methods; published in 2005.
- Schneider Electric: QO and QOB VH Interrupting Ratings — manufacturer confirmation of 22 kAIC VH versus standard 10 kAIC ratings.